In U.S. Pat. No. 4,818,480, issued Apr. 4, 1989 to L. E. Branovich et al, there is disclosed and claimed a method of making a long life high density cathode from a mixture of tungsten and iridium powders that uses as the impregnant barium peroxide alone, or a mixture of barium peroxide with iridium, or a mixture of barium peroxide with osmium, or a mixture of barium peroxide with rhodium. Also, in U.S. Pat. No. 4,840,767, issued June 20, 1989 to L. E. Branovich et al, there is described and claimed a method of making a long life high current density cathode from a mixture of tungsten and iridium powders using a barium iridiate formed from barium peroxide and iridium oxide as the impregnant. Although desirable in many aspects, the difficulty with the method of the inventions in these patents is that the chemical reaction is not sufficiently rapid and the resulting emission over a set time period is not sufficient.
The general object of this invention is to provide an improved method of making a long life high current density cathode. A more particular object of the invention is to provide such a method that will be rapid and in which the resulting emission over a set time period will be sufficient.
It has now been found that such a method can be provided by making such a cathode from a mixture of tungsten and iridium powders to form a porous billet and using a reaction product formed from reacting barium peroxide with an excess of tungsten as the impregnant for the porous billet.
By an excess of tungsten is meant that the molar ratio of barium peroxide to tungsten in forming the impregnant is less than 3 to 1. This is because in a 3:1 molar ratio of BaO.sub.2 to W, the resulting product formed is barium tungstate, Ba.sub.3 WO.sub.6 according to the reaction: EQU 3BaO.sub.2 +W.fwdarw.Ba.sub.3 WO.sub.6
If an excess of tungsten as for example an 0.1 molar excess of tungsten is added to the reactants, the resulting product is Ba.sub.3 WO.sub.6 with excess tungsten dispersed throughout the impregnant. Since the impregnant must react with the wall of the porous billet that is composed of tungsten or tungsten/iridium, the amount of surface area in the 3:1 molar BaO.sub.2 /W with 0.1 molar excess of tungsten is increased giving greater surface to surface contact of the impregnant to react with the tungsten within the impregnant as well as with the wall of the porous tungsten or tungsten/iridium impregnated billet. Therefore, a faster chemical reaction occures and there is greater emission over a set period of time.
A small amount of iridium may also be included in the impregnant. This may be about the same amount as the excess of tungsten included in the impregnant. Osmium or rhodium can be substituted in lieu of iridium. Moreover, it is also within the scope of the invention to substitute molybdenum or aluminum for part or all of the tungsten in the impregnant. It is also important that the average particle size of the impregnant powders be smaller than the average pore size of the billet.